The solution is simple, really.
Bury one of these bad boys somewhere:
http://www.rainharvest.com/norwesco-...k-102-wide.asp
Bam! 100% water change, whenever you like.
What's the frostline? It can't be more than 8 feet or so.
No need to thank me, I like to think of myself as a problem solver.
In all seriousness if there was anyone who could get away with a buried 2500 mixing tank somewhere, I would have guess that was you
Frankly I wish I could somehow get a buried 500g tank under my deck and be able to use it year-round. I totally empathize with the "want to intervene in an emergency". Well even without emergencies, there are times you just want to do a big/significant water change. I just changed out 50g out of my 65g FOWLR yesterday, you wouldn't believe how good it looks today. That same volume however is a "regular/weekly" waterchange for my 280g and when things aren't going right, it's really only a drop in the bucket. There are many times per year I find myself wistfully wishing I could changeout 150g-200g (or more) at a time, but I just simply don't know where I can put that much water to mix it up. So I do feel the pain of "sometimes a bigger tank isn't necessarily better" sentiment.